The present invention relates broadly to a method and apparatus for forming a headpiece on a preformed tubular body, and is more particularly concerned with providing a mold cavity in which to form a headpiece on a thin-walled preformed plastic or laminate tube, such as a bag intended for use in an aerosol can.
Various methods are used for forming a headpiece on a preformed tubular body. In one commonly-used method, a preformed tubular body is placed upon a poincon which had a diameter smaller than the inside diameter of the tubular body which allows a slight tolerance or clearance between the poincon and the tubular body. Then the poincon is inserted into an opposing female forming member such that the top end of the tubular body carried on the poincon extends beyond the head end of the poincon and abuts an inwardly curved inside surface of the opposing female member. The curved inside surface of the female member forms and bends or rolls the end of the tubular body inwardly effecting a tight sealing fit with the outside surface of the tubular body.
In order to prevent thermoplastic material from escaping through the tolerance or clearance between the inside surface of the tubular body and the poincon, or so-called "blow-up" of the molten material during the molding of the headpiece, it is necessary to maintain a tight sealing fit also between the poincon and the inside surface of the tubular body. Up to the present time, this has been accomplished by providing the inside surface of the female forming member, at a location directly opposite the top outer edge of the poincon, with a slightly smaller inside diameter than its overall inside diameter so that simultaneously as the poincon reaches the end of its insertion stroke this portion of the female member with the reduced diameter, together with the top outer edge of the poincon, will "squeeze" the tubular body to form a tight seal.
However, the above approach requires the maintenance of extremely precise tolerances and presents complications when one desires to automatically load preformed tubes onto the poincon, particularly, in the case of loading small tubes with thin-walled constructions, such as contemplated for use as bag elements in aerosol cans.
The present invention overcomes this problem by using a poincon, with a substantially undersized diameter relative to the inside diameter of the tube body, having an expandable washer at its head end. The use of this expandable washer, in conjunction with the undersized diameter of the poincon, enables a loose fitting installation of the tube body automatically onto the poincon prior to the headpiece forming operation.
The conically-shaped washer is normally in a contracted form, being retained by the bottom end of the cylindrical head of a soupape which is mounted for axial movement within the head end of the poincon. After the poincon with the tube body loosely fitted thereon is inserted into the female forming member and when it has moved therein toward its closed position relative to the female member at the end of its forward stroke, the top end of the cylindrical head of the soupape makes contact with the female member and depresses or retracts into the poincon as the poincon continues to advance forward to the end of the stroke. As the final advance of the poincon cause the soupape to retract, the head end of the advancing poincon expands the periphery of the washer to effect a peripheral contact of the washer with the inside diameter of the tube body establishing a tight sealing fit between the poincon, the tube body and the female forming member to define a mold cavity. While the forming elements are maintained in this position, molten thermoplastic material is introduced into the mold cavity to form the headpiece on the preformed tube body.